This invention relates generally to a liquid crystal electrooptical element that employs a chiral nematic liquid crystal having a bistable capability and, more particularly, to a liquid crystal electrooptical element capable of being multiplex driven employing the bistable quality of the element to perform the switching function.
Currently, the liquid crystal display devices employed as display devices for office equipment and machines, portable display devices and the like employ twisted nematic (TN) liquid crystal medium or supertwisted nematic (STN) liquid crystal medium. As an example, such liquid crystal display systems are described generally in the article of M. Schadt and W. Helfrich entitled, "Voltage-Dependent Optical Activity of A Twisted Nematic Liquid Crystal", Applied Physics Letters, Vol. 18(4), pp. 127-128 (1971) and in the article of T. J. Scheffer and J. Nehring "A New, Highly Multiplexable Liquid Crystal Display", Applied Physics Letters, Vol. 45(10), pp. 1021-1023 (Nov. 15, 1984). However, these mediums do not have a lasting stable memory effect. Therefore, these mediums need to be actively driven, e.g., by a multiplexing driving method using voltage averaging or by an active matrix driving method in which a transistor or other active element is provided at each picture element.
Other liquid crystal display systems are being currently researched and developed in addition to the foregoing, although they have not reached a reliable product stage. For example, high speed addressing technologies utilizing voltage averaging are disclosed in U.S. Pat. Nos. 4,566,758 and 4,582,396. LC systems utilizing bistable switching properties are disclosed in U.S. Pat. Nos. 4,239,345 and 4,505,548.
However, these disclosed LC systems inherently have several problems. When a twisted nematic liquid crystal is driven by voltage averaging, the ratio of the voltage V.sub.ON, for selecting the ON condition, to the voltage V.sub.OFF, for selecting the OFF condition, is given by the following equation, EQU V.sub.ON /V.sub.OFF =((N.sup.1/2 +1)/(N.sup.1/2 -1)).sup.1/2,
wherein N is the number of scan lines.
As can be seen from this equation, since V.sub.ON /V.sub.OFF approaches 1 as N becomes large, the contrast ratio also approaches 1. Considering both electrooptical characteristics of current liquid crystal materials and the delay experienced upon the application of voltage waveforms to such current liquid crystal materials, N is presently limited up to about 500 scan lines as a maximum. Therefore, it is impossible to employ this kind of system as display devices for use in workstations and the like requiring high definition displays. Also, the display characteristic is significantly dependent on the viewing angle, and, further, the switching time is comparatively too long.
The LC system disclosed in U.S. Pat. No. 4,582,396 is aimed at shortening the required switching time. By the disclosed addressing means in this publication, the switching from the ON condition to the OFF condition can be advanced by providing for opposite signs proscribed to the pretilt angle, .theta..sub.1, on the lower substrate, i.e., the angle formed between the molecular director of the liquid crystal molecules in contact with the alignment layer formed on the substrate surface, and the pretilt angle, .theta..sub.2, on the upper substrate. Also, U.S. Pat. No. 4,566,758 is aimed at stabilizing the operating condition by adding a chiral component to the liquid crystal medium. Even if, however, these technologies are utilized, bistable operation cannot be achieved and the liquid crystal electrooptical element must be driven by means of a voltage averaging method. As a result, there is no possibility of realizing a high definition liquid crystal display device through the utilization of these conventionally applied technologies.
A chiral nematic liquid crystal electrooptical element with bistable characteristics is disclosed in U.S. Pat. No. 4,239,345. This liquid crystal electrooptical element has a 180.degree. twist in its initial state, but after the voltage is applied, the element relaxes to two states different from its initial state, which are highly stable states. The difference in twist between the two states is approximately 360.degree., and bistable switching is performed between the two states. The two states can be optically distinguished by sandwiching the liquid crystal electrooptical element between two polarizing plates. In this situation, it should be noted that a high contrast ratio can be achieved by configuring the liquid crystal electrooptical element such that the result of dividing the product, R, (comprising the birefringence, .DELTA.n, of the liquid crystal medium and the layer thickness, d, (.mu.m) of the liquid crystal medium) by the wavelength, .lambda., of the incident light, i.e., R/.lambda., is less than 2.
The foregoing patent mentions a simple matrix driving system without detailing such a system. U.S. Pat. No. 4,505,548 discloses a system for relatively rapid bistable switching by controlling the applied voltage. However, the liquid crystal medium requires a rather high pretilt angle of 35.degree. so that the liquid crystal alignment film must be formed by means of oblique deposition to achieve such a high pretilt angle. However, this type of alignment has not been proven to be sufficiently practical for use in production.
Further, liquid crystal electrooptical elements utilized in display devices requiring a high pretilt angle and liquid crystal display devices driven at a high duty ratio by a voltage-averaging method provide a narrow effective viewing angle.
When a liquid crystal electrooptical element is employed as a display panel, normally a red, green and blue three-wavelength light source or white light source is employed. Therefore, the element must have a high contrast ratio and a high light transmittance in its ON state for those kinds of light sources. Therefore, it is necessary to know the conditions for obtaining good display characteristics when employing these kinds of light sources.
Parameters that determine good display characteristics include the birefringence, .DELTA.n, and the layer thickness, d, of the liquid crystal medium as well as the angles of the polarization axes of the two polarizing plates. U.S. Pat. No. 4,239,345 indicates only that, with respect to R (=d.multidot..DELTA.n), a high contrast ratio can be achieved by making R/.lambda..ltoreq.2 when a monochromatic light is employed as the light source. There is no recognition, appreciation or reference to the conditions required to achieve high ON light transmittance or to the necessary angle of the polarization axes. There is no discussion or appreciation at all of the conditions required when employing white light. Although mentioned is made that a high contrast ratio can be achieved by making R/.lambda..ltoreq.2, R/.lambda. in each of the disclosed embodiments is greater than 2, i.e., R/.lambda..gtoreq.2. Also, there is no data relating to the resulting electrooptical characteristics of light transmittance or the contrast ratio and a clear understanding and knowledge as to what requirements are necessary to consider in achieving these characteristics on a high level or, further, at to optimum level.
In experiments conducted by the inventors herein employing white light, they were able to optically distinguish the two metastable states, but we did not obtain display characteristics that would render the liquid crystal electrooptical element useful as a display panel device.
As will be described below, in a liquid crystal display which utilizes two metastable states in which the twist angle is o.sub.r .+-.180.degree. setting the polarization axes, which is one of the parameters that determines the display characteristics, to an optimum angle is not easy to achieve. In such a bistable liquid crystal display system, the twist angle is 180.degree. in the initial state, and one of the two metastable states is a uniform state with a twist angle of 0.degree.. Therefore, the light transmittance when the two polarizing plates are each disposed at a predetermined angle can be achieved relatively easily. However, the twist angle of the other metastable state is approximately 360.degree. and, in order to speed up the response time of the medium as much as possible, the liquid crystal layer must be formed as thin as possible. When the twist angle is large, such as 360.degree., and the liquid crystal layer made thin, the linearly polarized light (white light) incident on the liquid crystal medium is exited as elliptically polarized light, thus rendering it difficult to achieve the transmitted light spectrum in the 360.degree. twisted state. Thus, it has not been established and determined as to how the polarization axes should be set to increase both the contrast ratio and the ON transmittance when R is a predetermined value.
It is, therefore, an object of this invention to offer a bistable liquid crystal electrooptical element that employs a chiral nematic liquid crystal having a sufficiently good display characteristic as a display element with a high contrast ratio and a high ON transmittance.
In the liquid crystal electrooptical element of the invention, the respective stabilities of the initial state and the two metastable states are dependent on the ratio, d/p.sub.o, of the liquid crystal layer thickness, d, to the liquid crystal helical pitch, P.sub.o. Where the twist angle in the initial state is o.sub.r, the desired alignment state in the initial state can be realized by satisfying the condition, EQU 0.5o.sub.r /360.degree..ltoreq.d/p.sub.o .ltoreq.1.5o.sub.r /360.degree..
However, regarding the two metastable states obtained by applying a driving waveform, the metastable state having a twist angle of approximately, o.sub.r +180.degree., becomes more unstable compared to the other metastable state under the condition, EQU d/p.sub.o .apprxeq.0.5o.sub.r /360.degree.,
while under the condition, EQU d/p.sub.o .apprxeq.1.5o.sub.r /360.degree.,
the metastable state having a twist angle of approximately, o.sub.r +180.degree., becomes more unstable compared to the other metastable state. A liquid crystal display device having unbalanced stabilities relative to the two metastable states requires comparatively a larger amount of energy to select the unstable state, which has the undesirable problem of increasing the drive voltage or requiring a drive pulse voltage applied over a longer period of time. Further, since the time required for relaxation from a selected unstable metastable state to return to the initial state, which is referred to as the memory retention time, is short, relaxation may occur in a nonselection period in the case where display device contains a large number of scanning electrodes thereby degrading display quality.
The pretilt angle, formed between the planar extent at the substrate interface and the molecular director vector of the liquid crystal molecules at the interface, also has an effect on the balance achieved between the stabilities of the two metastable states. According to the results of experiments conducted by the inventors, they have discovered that the stability of the metastable state having a twist angle o.sub.r +180.degree. dropped as the pretilt angle became larger. Such a display has the same problems as described above. In addition, high pretilt angle will provide a narrow effective viewing angle.
Further, in a bistable liquid crystal display or in a multistable liquid crystal display, the alignment states employed for display of information are frequently not sufficiently stable, and plural states may occur at the same time due to local displacement in the surface condition or shape of the substrate resulting in poor display quality.
In a liquid crystal display utilized in accordance with this invention, a shape effect occurs along the edges of the transparent segment electrodes, i.e., the tapered edges along the length of the electrodes have an effect in selection of metastable states. This is illustrated in FIGS. 15 and 16. FIG. 15 schematically illustrates scan electrodes 54 formed on substrate 50B while signal electrodes 52 are formed on substrate 50A. These electrodes are formed in orthogonal relation, as seen in FIG. 16, defining pixel regions 60. As noted best in FIG. 15, pixel regions 60 include areas 58 plus areas 56 between electrodes 52 and 54. The regions between pixel regions 60 are nonpixel regions 62. Side edges 57 of electrodes 52 and 54 are not planar like surfaces 59 of electrodes 52 and 54, due to fabrication methods. As a result, the alignment state of the liquid crystal molecules in a region of areas 58 between surfaces 59 of electrodes 52 and 54 will be different from the affects on the liquid crystal medium in a region of areas 56 between edges 57 of electrodes 52 and 54, particularly since the alignment direction, n.sub.2, of the liquid crystal molecules at surfaces 59 of electrodes 52 and 54 will be different from the alignment direction, n.sub.1 and n.sub.2', of the liquid crystal molecules at tapered edges 57 of electrodes 52 and 54. As an example, when the twist angle o.sub.r +180.degree. is selected as the metastable state through application of an appropriate drive voltage waveform forming a domain having the twist angle o.sub.r +180.degree. above pixel regions 60, a domain having the twist angle o.sub.r -180.degree. concurrently appears in areas 56 immediately above the scan electrode tapered edges 57 so that the contrast ratio is reduced due to the result of this shape effect. This tendency is most evident at a selected longitudinal side portions of the pixels, comprising the intersections of signal electrodes 52 and scan electrodes 54, as illustrated in FIG. 16 by the stippling around the edge of pixel regions 60, as indicated at 64 and 66, when the rubbing direction is parallel to either the direction of the scan electrode or the direction of the signal electrode. In the case of FIG. 16, the rubbing direction is shown parallel in the direction of signal electrodes 52 so that this shape effect tendency appears most prominent in regions 66 and also appears to a lesser extent in regions 64. The utilization of the present invention has been able to substantially eliminate this shape effect.
Thus, it is an another object of this invention to offer a high definition liquid crystal display device having plural scan lines wherein the display characteristic of the bistable liquid crystal electrooptical element provides little dependence on the viewing angle.